This invention relates to electrical propulsion systems and more particularly to control arrangements for such systems.
In electrical propulsion systems for relatively large transport means, such as ships, effective arrangements must be provided for dissipation of regenerative energy developed in such systems, particularly during rapid deceleration of such transport means. For example, in connection with ships such as ferries, it is necessary to effect deceleration of an object having relatively high inertia as the ferry approaches its slip for docking. Usually, in connection with ship propulsion this deceleration is accomplished by reversing the propellers so as to slow the forward motion of the ship.
If the propellers are driven by electric motors the continuing motion of the ship thru the water as a result of this inertia tends to cause regenerative energy to be developed in the system as a result of the effect of the surrounding water on the propellers as the ship continues to move thru the water.
In some prior art propulsion systems, which have employed direct current generators as a source of power, it has been possible to dissipate this regenerative energy by causing the generator to motor the prime mover and thereby dissipate the energy as losses in the prime mover. In systems which employ an alternating current generator for driving direct current propulsion motors through an intervening rectifying means it is not possible to dissipate regenerative energy in the above manner, since the rectifying means blocks the transmission of such regenerative energy back to the generator and thence to the prime mover.
Another prior art arrangement for dissipating regenerative energy in connection with motor reversal in both marine and industrial applications is the use of resistors. The function of such resistors is to absorb and dissipate the regenerative energy as heat. In the case of large apparatus the size and cooling requirements of such resistors become a serious problem, particularly in shipboard installations where space may be limited.
In accordance with the present invention, an arrangement is provided which dissipates such regenerative energy in a system including an alternating current generator and rectifying means without requiring large banks of power resistors or other bulky heat dissipating apparatus. Moreover, particularly as applied to shipboard installations, the system of this invention provides rapid and smooth propeller reversal while limiting the current to a level below that which could damage the apparatus employed in the system.
It is an object of this invention to provide improved means for controlling the propulsion of a ship or other transport means having relatively high inertia.
It is another object of this invention to provide an improved electrical propulsion system which includes an improved arrangement for preventing overcurrent in the electrical propulsion system.
It is a further object of this invention to provide an improved means for effecting more rapid deceleration of the driven transport means.
It is still a further object of this invention to provide an improved electrical propulsion system including improved means for dissipating energy.
It is another object of this invention to provide, in a ship propulsion system, a more rapid and smoother propeller reversal while maintaining the current in the system below the level which could result in damage to the electrical apparatus employed.
In carrying out the objects of this invention, in one form thereof, an electrical propulsion system and control arrangements therefor are provided which include an alternating current generator which may be driven, for example, by a diesel engine. The output of the generator is connected to suitable rectifying means for converting the alternating current output to direct current. In the specific form of the invention disclosed two electrical motors are provided, each of which is connected to drive a propeller for moving the ship on which the equipment is mounted. The armatures of the motors are connected in series with the rectifying means in a closed loop. Each of the motors include a field whose polarity and magnitude are controlled for varying the speed and direction of rotation of the motor. Provision is also made for controlling the field of the generator to vary the output thereof.
When it is desired to reverse the direction of the motors, for example in effecting deceleration of the ship, the field of the motors are reversed. As a result of regenerative energy fed back into the system thru continued motion of the ship, the current in the aforementioned closed loop could reach an undesirably high value. The system includes a sensor for detecting when the current in the closed loop reaches a predetermined maximum value and for supplying a signal to regulating means for reducing power generated by the alternating current generator and for altering the polarity and magnitude of field current of one of the motors to maintain the current in the closed loop below the predetermined level. Energy fed back into the system as a result of the reversal of the field of one of the motors is dissipated into the water thru the propeller driven by the other motor. During any period of overcurrent in the closed loop, the field of this motor is automatically maintained at a polarity to drive that motor in its original direction, and the current therethrough is maintained at a magnitude sufficient to insure that the current in the closed loop does not exceed the predetermined maximum value.